Hydrolysis is one process for the decomposition of fluorine compounds, such as PFC and HFC. In this process, water is added to gaseous fluorine compounds to react them, and fluorine contained in the fluorine compounds is converted to hydrogen fluoride (HF), which is subsequently absorbed by water or an alkaline aqueous solution.
Japanese Patent Laid-Open No. 192653/1998 discloses a process, in which water vapor is added to a C2F6 gas or an NF3 gas, which are kinds of the fluorine compounds, and the fluorine compounds is made to contact with a catalyst containing at least one selected from alumina, titania, silica and zirconia at a temperature from 400° C. to 800° C. to hydrolyze the fluorine compounds. The publication discloses that when an oxidative gas, such as oxygen, is contained upon conducting hydrolysis, an oxidation reaction of carbon monoxide (CO), which is formed during the hydrolysis reaction, can be simultaneously induced. It also discloses that after removing hydrogen fluoride from a gas formed by the hydrolysis, the remaining gas may be made in contact with a CO oxidation catalyst to oxidize CO to CO2.
It is also known that an organic halogen compounds containing halogen, such as chlorine, fluorine and bromine, can be simultaneously subjected to both hydrolysis and oxidative decomposition as disclosed in Japanese Patent Laid-Open No. 122419/1992 and Japanese Patent Laid-Open No. 220346/1993. These publications specifically disclose decomposition processes of a chlorine compound such as Freon and PCB.
In the case where organic fluorine compounds such as PFC and HFC, is hydrolyzed, a CO gas is formed associated with hydrogen fluoride. Therefore, a CO gas is necessarily processed upon hydrolysis of an organic fluorine compounds. Japanese Patent Laid-Open No. 192653/1998 discloses that CO is oxidized to CO2 by containing an oxidative gas upon hydrolysis of fluorine compounds, or in alternative, CO is oxidized with a catalyst after removing hydrogen fluoride from a gas formed by hydrolysis. However, as a result of investigation conducted by the inventors, there is limitation in removal of CO only by containing oxygen or air upon hydrolysis of fluorine compounds. Furthermore, the process of contacting a gas formed by hydrolysis with a CO oxidation catalyst after removing hydrogen fluoride exhibits a poor efficiency because the high temperature gas formed by hydrolysis is cooled to room temperature for removing hydrogen fluoride and is then again heated.
The inventors have found that in the case where a fluorine compound such as SF6 and NF3, is decomposed over a catalyst, the same problem occurs as in the case of decomposition of PFC and HFC. Specifically, it has been found that when SF6 or NF3 is hydrolyzed, SO2F2 or N2O is by-produced together with hydrogen fluoride. These by-products are also preferably decomposed.
Japanese Patent Laid-Open No. 286439/1998 discloses a process for the decomposition of gaseous fluorine-containing compound such as CF4, C2F6, and C3F8, by contacting the compounds with molecular oxygen and water in the presence of a catalyst. There is a possibility of forming CO since the supplied amount of molecular oxygen is such an amount that is sufficient to convert a carbon content (carbon atoms) in the fluorine-containing compounds to carbon dioxide and carbon monoxide.
Japanese Patent Laid-Open No. 192653/1998, Japanese Patent Laid-Open No. 286439/1998, Japanese Patent Laid-Open No. 122419/1992 and Japanese Patent Laid-Open No. 220346/1993 noted in the foregoing fail to disclose by-production of SO2F2 or N2O upon hydrolysis of SF6 or NF3, and also fail to disclose a process for removing these substances.